Binding a list in @RequestParam

Question

I'm sending some parameters from a form in this way:

myparam[0]     : 'myValue1'
myparam[1]     : 'myValue2'
myparam[2]     : 'myValue3'
otherParam     : 'otherValue'
anotherParam   : 'anotherValue' 
...

I know I can get all the params in the controller method by adding a parameter like

public String controllerMethod(@RequestParam Map<String, String> params){
    ....
}

I want to bind the parameters myParam[] (not the other ones) to a list or array (anything that keeps the index order), so I've tried with a syntax like:

public String controllerMethod(@RequestParam(value="myParam") List<String> myParams){
    ....
}

and

public String controllerMethod(@RequestParam(value="myParam") String[] myParams){
    ....
}

but none of them are binding the myParams. Even when I add a value to the map it is not able to bind the params:

public String controllerMethod(@RequestParam(value="myParam") Map<String, String> params){
    ....
}

Is there any syntax to bind some params to a list or array without having to create an object as @ModelAttribute with a list attribute in it?

Thanks

Answer 1

One way you could accomplish this (in a hackish way) is to create a wrapper class for the List. Like this:

class ListWrapper {
     List<String> myList; 
     // getters and setters
}

Then your controller method signature would look like this:

public String controllerMethod(ListWrapper wrapper) {
    ....
}

No need to use the @RequestParam or @ModelAttribute annotation if the collection name you pass in the request matches the collection field name of the wrapper class, in my example your request parameters should look like this:

myList[0]     : 'myValue1'
myList[1]     : 'myValue2'
myList[2]     : 'myValue3'
otherParam    : 'otherValue'
anotherParam  : 'anotherValue'

Answer 2

Or you could just do it that way:

public String controllerMethod(@RequestParam(value="myParam[]") String[] myParams){
    ....
}

That works for example for forms like this:

<input type="checkbox" name="myParam[]" value="myVal1" />
<input type="checkbox" name="myParam[]" value="myVal2" />

This is the simplest solution :)

Answer 3

Subscribing what basil said in a comment to the question itself, if method = RequestMethod.GET you can use @RequestParam List<String> groupVal.

Then calling the service with the list of params is as simple as:

API_URL?groupVal=kkk,ccc,mmm

Answer 4

Arrays in @RequestParam are used for binding several parameters of the same name:

myparam=myValue1&myparam=myValue2&myparam=myValue3

If you need to bind @ModelAttribute-style indexed parameters, I guess you need @ModelAttribute anyway.

Answer 5

Just complementing what Donal Fellows said, you can use List with @RequestParam

public String controllerMethod(@RequestParam(value="myParam") List<ObjectToParse> myParam){
....
}

Hope it helps!

Answer 6

It wasn't obvious to me that although you can accept a Collection as a request param, but on the consumer side you still have to pass in the collection items as comma separated values.

For example if the server side api looks like this:

@PostMapping("/post-topics")
public void handleSubscriptions(@RequestParam("topics") Collection<String> topicStrings) {

    topicStrings.forEach(topic -> System.out.println(topic));
}

Directly passing in a collection to the RestTemplate as a RequestParam like below will result in data corruption

public void subscribeToTopics() {

    List<String> topics = Arrays.asList("first-topic", "second-topic", "third-topic");

    RestTemplate restTemplate = new RestTemplate();
    restTemplate.postForEntity(
            "http://localhost:8088/post-topics?topics={topics}",
            null,
            ResponseEntity.class,
            topics);
}

Instead you can use

public void subscribeToTopics() {

    List<String> topicStrings = Arrays.asList("first-topic", "second-topic", "third-topic");
    String topics = String.join(",",topicStrings);

    RestTemplate restTemplate = new RestTemplate();
    restTemplate.postForEntity(
            "http://localhost:8088/post-topics?topics={topics}",
            null,
            ResponseEntity.class,
            topics);
}

The complete example can be found here, hope it saves someone the headache :)